It is no particular surprise that U.S. President Barack Obama is on the verge of turning over a new leaf with Iran. After all, over the course of his presidency, Obama has repeatedly emphasized that he would like the United States and Iran to overcome their 35 years of estrangement. What is surprising, however, is how rapprochement has come about -- not through negotiations over the fate of Tehran’s nuclear program, but as a result of the battle against ISIS.

Tehran and Washington find themselves on the same side in the fight against the Islamic State of Iraq and al-Sham (ISIS), also called the Islamic State (IS), and there are already signs that they have been cooperating against the extremist group’s advance through Iraq. Although there is no guarantee that this will last for the duration of the war, such cooperation is clearly a positive step.

The United States and Iran both view ISIS as a significant threat to their own interests. An ISIS stronghold near the Iranian border would be a profound and immediate security threat to Tehran. For one, the Sunni jihadists of ISIS are openly disdainful of the Shia faith, the sect of Islam that the overwhelming majority of Iranians and the majority of Iraqis adhere to. The group is already in a sectarian war in Syria and Iraq, and Tehran must assume that it eventually plans on turning its attention to Iran.

Washington, for its part, has also concluded that ISIS poses a significant threat. If ISIS manages to create a safe haven in Iraq, it could use the territory to plan operations against the West, undermine Western allies in the region, and endanger oil shipments in the Persian Gulf. In the meantime, the group’s war against the Iraqi state also poses a danger to U.S. interests. Over the past decade, Washington has paid a high price in blood and treasure to create a stable and relatively friendly Iraq. The collapse of that state would be a humiliating defeat.

Although the United States and Iran have different visions for the future of Iraq, they share three major strategic goals there: protecting Iraq’s territorial integrity; preventing a sectarian civil war that could easily metastasize into the entire region; and defeating ISIS. There is also a precedent of tactical cooperation in Iraq between Tehran and Washington: In 2001, the two cooperated to dislodge the Taliban from Afghanistan.

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